menzel, what do you mean by "fragment varying out variable"? 'Varying' was used for the interpolators that link vertex/geometry with fragment shaders, but now the term is dropped altogether.
If you mean just the fragment shader outputs, then i'm for it. If vertex attribs and uniforms can be queried then fragment outputs should too, at least for consistency.
There are APIs (BindFragDataLocation, GetFragDataLocation) that accept those names as arguments but there is no way to find what the names are (except by parsing the shader text by hand).
I think this inconsistency should be addressed.

As for it's usefulness, it sure is useful. For example you can think of some wrapper library that stays between the application and the OpenGL. That wrapper does not know the names of the fragment output variables in the shaders that the application gives (its a separate piece of software) but it still may want to do something with them (depending on what is the purpose of the wrapper).